feat(widgets/paragraph): improve scrolling

Add optional callback to `Wrap` to compute the scroll offsets given the wrapped lines. This let
users compute the offsets dynamically.

examples/demo/ui.rs

@@ -274,7 +274,7 @@ where
             .fg(Color::Magenta)
             .add_modifier(Modifier::BOLD),
     ));
-    let paragraph = Paragraph::new(text).block(block).wrap(Wrap { trim: true });
+    let paragraph = Paragraph::new(text).block(block).wrap(Wrap::default());
     f.render_widget(paragraph, area);
 }
 

examples/paragraph.rs

@@ -81,20 +81,20 @@ fn main() -> Result<(), Box<dyn Error>> {
                 .style(Style::default().bg(Color::White).fg(Color::Black))
                 .block(create_block("Left, wrap"))
                 .alignment(Alignment::Left)
-                .wrap(Wrap { trim: true });
+                .wrap(Wrap::default());
             f.render_widget(paragraph, chunks[1]);
             let paragraph = Paragraph::new(text.clone())
                 .style(Style::default().bg(Color::White).fg(Color::Black))
                 .block(create_block("Center, wrap"))
                 .alignment(Alignment::Center)
-                .wrap(Wrap { trim: true })
+                .wrap(Wrap::default())
                 .scroll((scroll, 0));
             f.render_widget(paragraph, chunks[2]);
             let paragraph = Paragraph::new(text)
                 .style(Style::default().bg(Color::White).fg(Color::Black))
                 .block(create_block("Right, wrap"))
                 .alignment(Alignment::Right)
-                .wrap(Wrap { trim: true });
+                .wrap(Wrap::default());
             f.render_widget(paragraph, chunks[3]);
         })?;
 

examples/paragraph2.rs

@@ -0,0 +1,81 @@
+#[allow(dead_code)]
+mod util;
+
+use crate::util::event::{Event, Events};
+use std::{error::Error, io};
+use termion::{event::Key, input::MouseTerminal, raw::IntoRawMode, screen::AlternateScreen};
+use tui::{
+    backend::TermionBackend,
+    layout::{Alignment, Margin},
+    style::{Color, Style},
+    text::{Span, Spans},
+    widgets::{Block, Borders, Paragraph, Wrap},
+    Terminal,
+};
+
+const LOREM_IPSUM: &str = "Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua";
+
+fn main() -> Result<(), Box<dyn Error>> {
+    // Terminal initialization
+    let stdout = io::stdout().into_raw_mode()?;
+    let stdout = MouseTerminal::from(stdout);
+    let stdout = AlternateScreen::from(stdout);
+    let backend = TermionBackend::new(stdout);
+    let mut terminal = Terminal::new(backend)?;
+
+    let events = Events::new();
+
+    let mut i = 0;
+    let mut lines = Vec::with_capacity(100);
+    while i < 100 {
+        lines.push((i, format!("{}: {}", i, LOREM_IPSUM)));
+        i += 1;
+    }
+    loop {
+        terminal.draw(|f| {
+            let size = f.size();
+            let text: Vec<Spans> = lines
+                .iter()
+                .cloned()
+                .map(|(j, l)| {
+                    let span = if i == j + 1 {
+                        Span::styled(l, Style::default().bg(Color::Yellow))
+                    } else {
+                        Span::raw(l)
+                    };
+                    Spans::from(span)
+                })
+                .collect();
+            let mut wrap = Wrap::default();
+            wrap.scroll_callback = Some(Box::new(|text_area, lines| {
+                let len = lines.len() as u16;
+                (len.saturating_sub(text_area.height), 0)
+            }));
+            let paragraph = Paragraph::new(text)
+                .block(Block::default().borders(Borders::ALL))
+                .wrap(wrap)
+                .alignment(Alignment::Left);
+            f.render_widget(
+                paragraph,
+                size.inner(&Margin {
+                    vertical: 2,
+                    horizontal: 2,
+                }),
+            );
+        })?;
+
+        match events.next()? {
+            Event::Tick => {
+                lines.push((i, format!("{}: {}", i, LOREM_IPSUM)));
+                lines.remove(0);
+                i += 1;
+            }
+            Event::Input(key) => {
+                if key == Key::Char('q') {
+                    break;
+                }
+            }
+        }
+    }
+    Ok(())
+}

examples/popup.rs

@@ -81,12 +81,12 @@ fn main() -> Result<(), Box<dyn Error>> {
 
             let paragraph = Paragraph::new(text.clone())
                 .block(Block::default().title("Left Block").borders(Borders::ALL))
-                .alignment(Alignment::Left).wrap(Wrap { trim: true });
+                .alignment(Alignment::Left).wrap(Wrap::default());
             f.render_widget(paragraph, chunks[0]);
 
             let paragraph = Paragraph::new(text)
                 .block(Block::default().title("Right Block").borders(Borders::ALL))
-                .alignment(Alignment::Left).wrap(Wrap { trim: true });
+                .alignment(Alignment::Left).wrap(Wrap::default());
             f.render_widget(paragraph, chunks[1]);
 
             let block = Block::default().title("Popup").borders(Borders::ALL);

src/widgets/paragraph.rs

@@ -40,9 +40,8 @@ fn get_line_offset(line_width: u16, text_area_width: u16, alignment: Alignment)
 ///     .block(Block::default().title("Paragraph").borders(Borders::ALL))
 ///     .style(Style::default().fg(Color::White).bg(Color::Black))
 ///     .alignment(Alignment::Center)
-///     .wrap(Wrap { trim: true });
+///     .wrap(Wrap::default());
 /// ```
-#[derive(Debug, Clone)]
 pub struct Paragraph<'a> {
     /// A block to wrap the widget in
     block: Option<Block<'a>>,
@@ -70,7 +69,7 @@ pub struct Paragraph<'a> {
 ///     - Here is another point that is long enough to wrap"#);
 ///
 /// // With leading spaces trimmed (window width of 30 chars):
-/// Paragraph::new(bullet_points.clone()).wrap(Wrap { trim: true });
+/// Paragraph::new(bullet_points.clone()).wrap(Wrap::default());
 /// // Some indented points:
 /// // - First thing goes here and is
 /// // long so that it wraps
@@ -78,19 +77,30 @@ pub struct Paragraph<'a> {
 /// // is long enough to wrap
 ///
 /// // But without trimming, indentation is preserved:
-/// Paragraph::new(bullet_points).wrap(Wrap { trim: false });
+/// Paragraph::new(bullet_points).wrap(Wrap { trim: false, ..Wrap::default() });
 /// // Some indented points:
 /// //     - First thing goes here
 /// // and is long so that it wraps
 /// //     - Here is another point
 /// // that is long enough to wrap
 /// ```
-#[derive(Debug, Clone, Copy)]
 pub struct Wrap {
     /// Should leading whitespace be trimmed
     pub trim: bool,
+    pub scroll_callback: Option<Box<ScrollCallback>>,
 }
 
+impl Default for Wrap {
+    fn default() -> Wrap {
+        Wrap {
+            trim: true,
+            scroll_callback: None,
+        }
+    }
+}
+
+pub type ScrollCallback = dyn FnOnce(Rect, &[(Vec<StyledGrapheme<'_>>, u16)]) -> (u16, u16);
+
 impl<'a> Paragraph<'a> {
     pub fn new<T>(text: T) -> Paragraph<'a>
     where
@@ -130,6 +140,42 @@ impl<'a> Paragraph<'a> {
         self.alignment = alignment;
         self
     }
+
+    fn draw_lines<'b, T>(
+        &self,
+        text_area: Rect,
+        buf: &mut Buffer,
+        mut line_composer: T,
+        scroll: (u16, u16),
+    ) where
+        T: LineComposer<'b>,
+    {
+        let mut y = 0;
+        let mut i = 0;
+        while let Some((current_line, current_line_width)) = line_composer.next_line() {
+            if i >= scroll.0 {
+                let cell_y = text_area.top().saturating_add(y);
+                let mut x = get_line_offset(current_line_width, text_area.width, self.alignment);
+                for StyledGrapheme { symbol, style } in current_line {
+                    buf.get_mut(text_area.left() + x, cell_y)
+                        .set_symbol(if symbol.is_empty() {
+                            // If the symbol is empty, the last char which rendered last time will
+                            // leave on the line. It's a quick fix.
+                            " "
+                        } else {
+                            symbol
+                        })
+                        .set_style(*style);
+                    x += symbol.width() as u16;
+                }
+                y += 1;
+            }
+            i += 1;
+            if y >= text_area.height {
+                break;
+            }
+        }
+    }
 }
 
 impl<'a> Widget for Paragraph<'a> {
@@ -158,40 +204,54 @@ impl<'a> Widget for Paragraph<'a> {
                 // composers to operate on lines instead of a stream of graphemes.
                 .chain(iter::once(StyledGrapheme {
                     symbol: "\n",
-                    style: self.style,
+                    style,
                 }))
         });
 
-        let mut line_composer: Box<dyn LineComposer> = if let Some(Wrap { trim }) = self.wrap {
-            Box::new(WordWrapper::new(&mut styled, text_area.width, trim))
-        } else {
-            let mut line_composer = Box::new(LineTruncator::new(&mut styled, text_area.width));
-            if let Alignment::Left = self.alignment {
-                line_composer.set_horizontal_offset(self.scroll.1);
-            }
-            line_composer
-        };
-        let mut y = 0;
-        while let Some((current_line, current_line_width)) = line_composer.next_line() {
-            if y >= self.scroll.0 {
-                let mut x = get_line_offset(current_line_width, text_area.width, self.alignment);
-                for StyledGrapheme { symbol, style } in current_line {
-                    buf.get_mut(text_area.left() + x, text_area.top() + y - self.scroll.0)
-                        .set_symbol(if symbol.is_empty() {
-                            // If the symbol is empty, the last char which rendered last time will
-                            // leave on the line. It's a quick fix.
-                            " "
-                        } else {
-                            symbol
-                        })
-                        .set_style(*style);
-                    x += symbol.width() as u16;
+        match self.wrap {
+            None => {
+                let mut line_composer = LineTruncator::new(&mut styled, text_area.width);
+                if let Alignment::Left = self.alignment {
+                    line_composer.set_horizontal_offset(self.scroll.1);
                 }
+                self.draw_lines(text_area, buf, line_composer, self.scroll);
             }
-            y += 1;
-            if y >= text_area.height + self.scroll.0 {
-                break;
+            Some(Wrap {
+                trim,
+                scroll_callback: None,
+            }) => {
+                let line_composer = WordWrapper::new(&mut styled, text_area.width, trim);
+                self.draw_lines(text_area, buf, line_composer, self.scroll);
+            }
+            Some(Wrap {
+                trim,
+                ref mut scroll_callback,
+            }) => {
+                let mut line_composer = WordWrapper::new(&mut styled, text_area.width, trim);
+                let mut lines = Vec::new();
+                while let Some((current_line, current_line_width)) = line_composer.next_line() {
+                    lines.push((Vec::from(current_line), current_line_width));
+                }
+                let f = scroll_callback.take().unwrap();
+                let scroll = f(text_area, lines.as_ref());
+                self.draw_lines(text_area, buf, WrappedLines { lines, index: 0 }, scroll);
             }
+        };
+    }
+}
+
+struct WrappedLines<'a> {
+    lines: Vec<(Vec<StyledGrapheme<'a>>, u16)>,
+    index: usize,
+}
+
+impl<'a> LineComposer<'a> for WrappedLines<'a> {
+    fn next_line(&mut self) -> Option<(&[StyledGrapheme<'a>], u16)> {
+        if self.index >= self.lines.len() {
+            return None;
         }
+        let (line, width) = &self.lines[self.index];
+        self.index += 1;
+        Some((&line, *width))
     }
 }

tests/widgets_paragraph.rs

@@ -25,7 +25,7 @@ fn widgets_paragraph_can_wrap_its_content() {
                 let paragraph = Paragraph::new(text)
                     .block(Block::default().borders(Borders::ALL))
                     .alignment(alignment)
-                    .wrap(Wrap { trim: true });
+                    .wrap(Wrap::default());
                 f.render_widget(paragraph, size);
             })
             .unwrap();
@@ -91,7 +91,7 @@ fn widgets_paragraph_renders_double_width_graphemes() {
             let text = vec![Spans::from(s)];
             let paragraph = Paragraph::new(text)
                 .block(Block::default().borders(Borders::ALL))
-                .wrap(Wrap { trim: true });
+                .wrap(Wrap::default());
             f.render_widget(paragraph, size);
         })
         .unwrap();
@@ -123,7 +123,7 @@ fn widgets_paragraph_renders_mixed_width_graphemes() {
             let text = vec![Spans::from(s)];
             let paragraph = Paragraph::new(text)
                 .block(Block::default().borders(Borders::ALL))
-                .wrap(Wrap { trim: true });
+                .wrap(Wrap::default());
             f.render_widget(paragraph, size);
         })
         .unwrap();